The present application is related to and claims priority from Japanese Patent Application No. 11-319599, filed Nov. 10, 1999.
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device which is provided with a novel spacer arrangement for keeping constant the distance between a pair of substances which seal therebetween liquid crystal compounds which constitute a liquid crystal layer.
Liquid crystal display devices are widely used as display devices capable of providing high-resolution and color display for notebook type computers or computer monitors.
These kinds of liquid crystal display device basically include a so-called liquid crystal panel in which liquid crystal compounds are interposed between at least two opposed substrates at least one of which is made of transparent glass or the like, and are generally divided into a type using a liquid crystal panel (simple matrix type: STN type) which selectively applies voltages to various kinds of pixel-forming electrodes formed over the substrates of the liquid crystal panel and turns on and off predetermined pixels, and a type using a liquid crystal panel(active matrix type) in which the various kinds of electrodes and pixel-selecting active elements are formed to turn on and off predetermined pixels by making selection from these active elements.
The active matrix type liquid crystal display devices are represented by a type which uses thin film transistors (TFTs) as active elements which constitute its liquid crystal panel. Liquid crystal display devices using thin film transistors have been widely spread as monitors for display terminals of OA equipment because of their thin sizes and light weights as well as their high picture qualities which compare with those of Braun tubes.
Liquid crystal panels for use in such an active matrix type liquid crystal display device are generally divided into the following two types on the basis of the difference between their liquid crystal driving methods. One of the types includes liquid crystal compounds interposed between two substrates on which transparent electrodes are formed, and the liquid crystal compounds are operated by voltages applied to the transparent electrodes and light which passes through the transparent electrodes and enters the layer of the liquid crystal compounds is modulated to display a picture (TN type).
The other type is constructed to operate a liquid crystal by an electric field which is formed between two electrodes formed on the same substrate, nearly in parallel with the surface thereof, and modulate light which enters the layer of liquid crystal compounds through the gap between the two electrodes, thereby displaying a picture. This type of liquid crystal display device has the feature of a remarkably wide viewing angle and is an extremely promising active matrix type liquid crystal display device. The feature of this type is described in, for example, International Patent Publication No. 505247/1993, Japanese Patent Publication No. 21907/1988 and Japanese Patent Laid-Open No. 160878/1994. This type of liquid crystal display device will be hereinafter referred to as the lateral electric field type (ISP type) of liquid crystal display device.
FIG. 14 is a diagrammatic cross-sectional view illustrating the essential portion of an example of the construction of a TN type of liquid crystal panel. In FIG. 14, symbols SUB1 and SUB2 denote transparent glass substrates. Thin film transistors TFT are formed over the inner surface of the transparent substrate SUB1, while color filters FIL (FIL(R), FIL(G) and FIL(B)) for three colors: red, green and blue are formed over the inner surface of the transparent substrate SUB2. Incidentally, the blue filter FIL(B) is not shown.
The thin film transistors TFT formed over the transparent substrate SUB1 are each made of a gate electrode GT, a gate insulating film GI, an insulating film AOF, a semiconductor layer AS, a drain electrode SD2 and a source electrode SD1, and a pixel electrode ITO1 is connected to the source electrode SD1. Incidentally, two thin film transistors TFT are formed per pixel for the purpose of improving the yield factor of products, but FIG. 14 shows one of the thin film transistors TFT (TFT1).
A protective film PSV1 is deposited as a layer which overlies these thin film transistors TFT, and an alignment layer (alignment control layer) ORI1 which is in contact with a liquid crystal (liquid crystal layer) LC is formed as the uppermost layer.
The shown liquid crystal panel has a light shield film, i.e., a so-called black matrix BM, at the boundary between each of the three color filters FIL (FIL(R), FIL(G) and FIL(B)) which are formed over the inner surface of the transparent substrate SUB2, and a protective film PSV2 is formed as a layer which overlies the black matrix BM. A common electrode (also called a counter electrode) ITO2 is formed over the protective film PSV2, and an alignment layer (alignment control layer) ORI2 which is in contact with the liquid crystal (liquid crystal layer) LC is formed as the uppermost layer. Polarizers POL1 and POL2 are stacked on the outer surfaces of the respective substrates SUB1 and SUB2.
This type of liquid crystal panel turns on/off each pixel by changing the alignment direction of the liquid crystal LC by means of an electric field generated between the common electrode ITO2 and the pixel electrode ITO1 which is driven by the thin film transistor TFT.
FIG. 15 is a diagrammatic cross-sectional view illustrating the essential portion of one example of the construction of an IPS type of liquid crystal panel. In FIG. 15, the same symbols as those shown in FIG. 14 denote the same functional portions as those shown in FIG. 14. In this liquid crystal panel, a video signal line (drain line) DL, a counter electrode CT (which corresponds to the common electrode ITO2 shown in FIG. 14), and a pixel electrode PX (which corresponds to the pixel electrode ITO1 shown in FIG. 14) are formed over one transparent substrate SUB1, and the alignment control layer ORI1 is formed at the interface between the layer of the liquid crystal LC and the protective film PSV which is deposited over the video signal line DL, the counter electrode CT and the pixel electrode PX. Color filters FIL for three colors (the three color filters are generally denoted by the common symbol FIL) which are separated from one another by the black matrix BM are formed over the other transparent substrate SUB2, and an overcoat layer OC (which corresponds to the protective film PSV2 shown in FIG. 14) is deposited to cover the black matrix BM and the color filters FIL so that the constituent materials of the color filters FIL and the black matrix BM are prevented from affecting liquid crystal compounds which constitute the liquid crystal LC. The alignment control layer ORI2 is formed at the interface between the overcoat layer OC and the liquid crystal LC.
The gate insulating film GI and the insulating film AOF which overlie the transparent substrate SUB1 are made of an insulating film, and the video signal line (drain line) DL is made of two layers: conductive films d1 and d2. The counter electrode CT is made of a conductive film g1, and the pixel electrode PX is made of a conductive film g2.
This IPS type of liquid crystal panel turns on/off each pixel by controlling the alignment direction of liquid crystal molecules by means of an electric field (shown as lines of electric force in FIG. 15) which is generated in a lateral direction (in a direction parallel to the substrates SUB1 and SUB2) between the pixel electrode PX and the counter electrode CT.
Incidentally, it is general practice to restrict the distance between the pair of substrates SUB1 and SUB2 (or the thickness of the layer of the liquid crystal compounds, or the gap between both substrates SUB1 and SUB2; hereinafter referred to as the cell gap or simply as the gap) to a predetermined value by disposing spherical spacers or spacers (not shown) between both substrates SUB1 and SUB2. The polarizers POL1 and POL2 are disposed on the outer surfaces of the respective substrates SUB1 and SUB2 in a manner similar to that shown in FIG. 14.
Although not related to the lateral electric field type of liquid crystal display device, Japanese Patent Laid-Open No. 73088/1997 discloses a liquid crystal display device in which, instead of such spherical spacers, conic spacers are formed on the protective film of a color filter substrate in such a manner as to be secured to the color filter substrate, or cylindrical spacers are fixedly formed on stacked color filter layers.
In the invention disclosed in the above-cited Japanese Patent Laid-Open No. 73088/1997, the spacers are formed in the state of being fixed to the substrate to solve problems which are experienced with the spherical spacers, such as a decrease in contrast due to light leaks from the peripheral portions of spherical spacers, and a display defect caused by spherical spacers being nonuniformly arranged in the process of scattering the spacers on the substrate.
Moreover, Japanese Patent Laid-Open No. 48636/1998 discloses IPS type TFT LCD has spacers in the intersection part of drain line and gate line.
Japanese Patent Laid-Open No. 325298/1995 discloses a method of forming spacers which hold the gap between substrates. This method uses a photolithographic process which stacks on a substrate a photosensitive sheet made of a base film coated with a photosensitive resin and executes exposure through a mask as well as development. This method is intended to make uniform the thickness of the spacers and prevent color irregularity.
As other prior arts, there are Japanese Patent Laid-Open No. 173104/1993 and No. 173148/1993 having bead in the display area.
In a liquid crystal display device using any of the above-described various kinds of liquid crystal panels, the gap between its substrates is restricted by a polymer-made bead (granular bead) or a columnar bead. In recent years, such a liquid crystal display device has been adopted not only in comparatively small-sized and light-weight electronic equipment such as notebook type personal computers, but also as a so-called display monitor, so that the liquid crystal display device has been becoming larger in screen size.
If the size of a liquid crystal panel (the size of a screen) becomes large, in a display device of the type which is normally used in vertical position, particularly in a display monitor, the following phenomenon may occur: the liquid crystal interposed between its substrates descends due to its own weight and the gap on the lower side of the screen becomes large, so that display irregularity occurs. Such a phenomenon is remarkable in IPS and STN types of liquid crystal panels. In addition, in a liquid crystal panel of the type which uses polymer beads as its gap restriction member, there are some cases where the polymer beads are concentrated in a portion and the gap locally increases or a so-called light leak occurs, i.e., light passes through the polymer beads which are generally transparent, so that the picture quality of the liquid crystal panel is degraded.
A liquid crystal panel in which spacers are fixedly formed on one of its substrate instead of the polymer beads is known as an approach to solving such a problem. However, when the liquid crystal panel which uses such spacers is in use, if the expansion of its liquid crystal due to a temperature rise (in general, due to heat radiated from a backlight) occurs, the gap restriction ability of the spacers cannot follow the enlargement of the gap, so that the spacers may separate from the opposed substrate. As a result, gap irregularity occurs and causes picture quality irregularity.
In the liquid crystal panel of the type which uses polymer beads for the restriction of the gap, since multiple polymer beads are interposed between the substrates in a compressively deformed state during the step of ensuring the gap of the liquid crystal panel after the multiple polymer beads have been scattered between the substrates, the gap restriction ability can follow the above-described enlargement of the gap to some extent. However, since more than a predetermined number of polymer beads are needed in order to ensure the required gap, the concentration of the polymer beads as well as the occurrence of light leak can only be restrained within limits. In addition, as described above, there is the problem that the polymer beads descend with time toward the lower side of the liquid crystal panel due to their own weights during the repetition of the thermal expansion of the liquid crystal panel, or along with a descend of the liquid crystal.
The present invention solves the above-described problems of the related art and provides a liquid crystal display device which can display a high-quality picture by preventing gap irregularity due to a variation in the temperature of a liquid crystal panel.
According to one aspect of the present invention, spacer and bead fixedly formed on at least one of a pair of substrates which constitute a liquid crystal panel and a small number of polymer beads are used together as members which restrict the gap between the pair of substrates.
Although the construction of the present invention slightly differs among liquid-crystal-panel driving schemes (STN, TN and IPS), the basic construction in which both spacers and a small number of polymer beads are used together is common to any type of liquid crystal panel. Representative aspects of the present invention will be described below.
According to an aspect of the present invention, a liquid crystal display device includes: a liquid crystal panel having a pair of substrates, at least two or more kinds of color filters formed over one of a pair of substrates, a black matrix interposed between each of a color filters in matrix shape, a liquid crystal layer interposed between a pair of substrates, a spacer formed on at least one of a pair of substrates, and a bead interposed between a pair of substrates, wherein a spacer is disposed under a black matrix, and the height of a bead is greater than the height of a spacer.
In the aspect, the gap between the pair of substrates is mainly restricted by the height of the spacer, and when the liquid crystal display device is in use, if the spacer is separated from a substrate due to the expansion of the liquid crystal due to a temperature rise or the like and the gap is enlarged, the spacer is restored in a direction in which compressive deformation of a bead is released, thereby restricting the gap. Thus, the gap irregularity of its screen is reduced, and since the bead is maintained in contact with the substrate, the bead is prevented from traveling or descending due to its own weight and the liquid crystal is also restrained from traveling or descending, whereby the display quality of the liquid crystal display device is prevented from being extremely degraded.
Since the spacer is formed in the area covered with the black matrix, the spacer does not affect the transmission of light from a backlight. In addition, since only a small number of beads are needed, the phenomenon of light leak can be restrained to the minimum and a decrease in contrast can be restrained.
Moreover, the spacer is formed of the resin material by a photolithographic technique. Since the spacer is formed directly on the protective film as described above in the fourth aspect, the spacer can be firmly secured to the substrate which the spacer is formed. The substrate on which the spacer is formed is not limited to the substrate having the color filters, and the spacer can also be formed on the other substrate (in an active matrix type, a substrate on which thin film transistors are formed).
In a case where an electrode overlies the aforesaid underlying protective film, an opening is formed in the electrode and the spacer is connected to the protective film which underlies the electrode through the opening, whereby the spacer can be firmly secured to the substrate on which the spacer is formed.
Incidentally, the present invention is not limited to any of the above-described aspects, and spacers each having a height equivalent to half of the required gap may be formed, respectively, on a pair of substrates in opposition to each other.
Various modifications of the present invention can be made without departing from the technical ideas described in the appended claims.